This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Mouse early oocytes are transiently polar: three-dimensional and ultrastructural analysis. The oocytes of many invertebrate and non-mammalian vertebrate species are not only asymmetrical but also polar in the distribution of organelles, localized RNAs and proteins, and the oocyte polarity dictates the patterning of the future embryo. Polarly located within the oocytes of many species is the Balbiani body (Bb), which in Xenopus is known to be associated with the germinal granules responsible for the determination of germ cell fate. In contrast, in mammals, it is widely believed that the patterning of the embryo does not occur before implantation, and that oocytes are non-polar and symmetrical. Although the oocytes of many mammals, including mice and humans, contain Bbs, it remains unknown how and if the presence of Bbs relates to mouse oocyte and egg polarity.